Diffusion

Gas Diffusion

• Defined as the movement of gas from higher to lower partial pressure.
• Gradients for O2 and CO2 must be evaluated separately.

Dalton’s Law of Partial Pressures

• Each gas in a mixture has its own partial pressure.
• Ambient air: ~21% O2, ~78% N2.
• Barometric pressure at sea level: 760 mm Hg.

Calculation of Partial Pressure

• Formula: $(P<em>B - 47 ext{ mm Hg}) \times F</em>{gas} = P_{gas}$
• Example: For O2 in Boise ($P_B = 690 ext{ mm Hg}$):
  • $P_{IO2} = (690 - 47) \times 0.21 = 135 ext{ mm Hg}$

Alveolar Air Equation

• Modified equation: $(P<em>B - 47) \times F</em>{IO2} - \frac{PaCO2}{0.8}$
• Clinical assumption: $PaCO2 \approx PACO2$.

Fick’s Law of Diffusion

• Factors: A (surface area), D (diffusion constant), T (thickness); influx formula: $T A \times D \times (P<em>1 - P</em>2)$.
• Higher thickness or lower surface area limits diffusion rate.

Capillary Transit Time

• Normal: 0.25 seconds required for O2 saturation, allowed time: 0.75 seconds.
• Oxygenation may decrease with conditions that thicken diffusion membranes.

Exercise and Disease Effects

• Exercise: increased cardiac output, potential limitation on oxygenation.
• Disease: thickening of membranes can cause oxygenation issues during exercise.

Diffusion Barriers for O2

• Components: Surfactant, alveolar epithelium, basement membrane, capillary endothelium, RBC membrane.

Influence of Conditions on Diffusion

• Emphysema: impairs diffusion.
• Elite athletes: risk of mild pulmonary edema affecting PaO2.

Interactions During Ventilation and Diffusion Changes

• Increased dead space (VD) decreases alveolar ventilation (VA), raises PaCO2, and lowers PAO2, affecting diffusion gradients.